Field of the Invention
The present invention generally relates to the production of supported catalysts, and more particularly, to a method for making a heterogeneous catalyst containing metal nanoparticles dispersed throughout the support material for the heterogeneous catalyst.
Description of the Related Art
Many industrial products such as fuels, lubricants, polymers, fibers, drugs, and other chemicals would not be manufacturable without the use of catalysts. Catalysts are also essential for the reduction of pollutants, particularly air pollutants created during the production of energy and by automobiles. Many industrial catalysts are composed of a high surface area support material upon which chemically active metal nanoparticles (i.e., nanometer sized metal particles) are dispersed. The support materials are generally inert, ceramic type materials having surface areas on the order of hundreds of square meters/gram. This high specific surface area usually requires a complex internal pore system. The metal nanoparticles are deposited on the support and dispersed throughout this internal pore system, and are generally between 1 and 100 nanometers in size.
Processes for making supported catalysts go back many years. One such process for making platinum catalysts, for example, involves the contacting of a support material such as alumina with a metal salt solution such as chloroplatinic acid. The salt solution “impregnates” or fills the pores of the support during this process. Following the impregnation, the support containing the salt solution would be air dried, causing the metal salt to precipitate within the pores. The support containing the crystallized metal salt would then be exposed to a hydrogen or carbon monoxide gas environment, reducing the solid metal salt to metal particles.
Another process for making supported catalysts involves the steps of contacting a support material with a metal salt solution and reducing the metal ions to metal particles in situ using suitable reducing agents. The following are examples of this process. U.K. Patent No. 1,282,138 teaches methods for preparing metal catalysts using different reducing agents, including glucose, hydroxylamine hydrochloride, and hydrazine. U.S. Pat. No. 4,086,275 teaches methods for preparing copper catalysts using sodium borohydride as the in situ reducing agent. U.S. Pat. No. 4,835,131 teaches methods for preparing molybdenum on silica catalyst, copper on gamma-alumina catalyst, silver on silica catalyst and silver on gamma-alumina catalyst. The reducing agents used to prepare these catalysts include hydrazine, ammonium hydroxide, and formaldehyde. U.S. Pat. Nos. 5,275,998 and 5,275,999 teach methods for preparing metal catalysts on carbon support and on alumina support using different reducing agents, including hydrazine hydrate, ascorbic acid, and sodium borohydride. According to these patents, supported catalysts having very small metal particle size (average size not greater than 2 nanometers) can be produced if the preparation steps are carried out in the presence of ethylene and/or acetylene (U.S. Pat. No. 5,275,998) or in the presence of carbon monoxide (U.S. Pat. No. 5,275,998). U.S. Pat. No. 6,686,308 teaches methods for preparing metal catalysts on silica using sodium citrate or potassium citrate as the reducing agent. This patent also teaches the use of colloid stabilizers including sodium sulfanilate, and discloses that it is preferable to use colloid stabilizers that can also act as reducing agents, namely ammonium citrate, potassium citrate, and sodium citrate.